Method and apparatus for bipolar servo marks writing with high output

ABSTRACT

A method and apparatus for bipolar servo marks writing with high output. The method includes providing a servo write head having first and second write gaps; and applying a varying current comprising either (i) abrupt changes in current in combination with current ramps or (ii) patterns of high-frequency current pulses before and after writing servo marks to a medium proximate to and moving past the first and second write gaps. The apparatus includes a bipolar servo write driver configured to generate a varying bipolar current signal having both negative and positive polarity currents and a servo write head having a first write gap and second write gap spaced apart, the varying current signal comprising either (i) abrupt changes in current in combination with current ramps or (ii) patterns of high-frequency current pulses before and after low-frequency pulses and a component to move a medium proximate to and past the first and second write gaps.

FIELD OF THE INVENTION

The present invention relates to the field of magnetic medium recording;more specifically, it relates to apparatuses and methods for bipolarwriting servo marks to magnetic recording medium.

BACKGROUND

Magnetic recording systems (such as tape recording systems) record bitsof information on a magnetic medium using a write/read head composed ofwrite and read transducers. During both write and read operations, therecording heads need to be positioned accurately relative to recordingdata tracks of the magnetic medium. This is achieved by controlling theposition of the write/read head in reference to servo marks prewrittenon the magnetic medium. The position accuracy of the write/read dataelements relative to data tracks strongly depends on the amplitude ofthe servo readback system. Present methods of writing servo marksincrease readback amplitude at the cost of introducing undesirablepulses into the readback signal. These undesirable pulses can result inpoor positioning of the read/write head causing data read errors.Alternatively, present methods of writing servo marks requirepre-erasure of the servo tracks, which add another step into the servomark writing process. Accordingly, there exists a need in the art tomitigate the deficiencies and limitations described hereinabove.

SUMMARY

A first aspect of the present invention is a method comprising:providing a servo write head having a first write gap and a second writegap spaced apart and an induction coil configured to generate respectivemagnetic fields proximate to the first and the second write gaps when acurrent is applied to the coil by a bipolar servo write driver, thebipolar servo write driver configured to generate both negative andpositive polarity currents; generating a varying current signal usingthe bipolar servo write driver; moving a magnetic storage medium pastthe first and the second gaps in a direction from the first write gaptoward the second write gap; the varying current signal includes anon-write phase, a preamble phase, a servo mark write phase and aclosing phase; the varying current at an initial current level duringthe non-write phase; the preamble phase comprising an abrupt change incurrent from the initial current level to a first current level and afirst current ramp from the first current level to a second currentlevel; the servo mark write phase comprising current pulses betweenthird and fourth current levels to write servo marks; the closing phasecomprising a second current ramp from the second current level to thefirst current level and an abrupt change in current from the firstcurrent level to the initial current level; and wherein servo marks arewritten only during the servo mark write phase.

A second aspect of the present invention is a method, comprising:providing a servo write head having a first write gap and a second writegap spaced apart and an induction coil configured to generate respectivemagnetic fields proximate to the first and the second gaps when acurrent is applied to the coil by a bipolar servo write driver, thebipolar servo write driver configured to generate both negative andpositive polarity currents; generating a varying current signal usingthe bipolar servo write driver; moving a magnetic storage medium pastthe first and the second write gaps in a direction from the first writegap toward the second write gap; the varying current signal comprises anon-write phase, preamble phase, a servo mark write phase and a closingphase; during the non-write phase, the varying current is at an initialcurrent level; the preamble phase comprises a preamble pattern ofhigh-frequency current pulses between a first current level and a secondcurrent level and back to the first current level; the servo mark writephase comprises a servo mark pattern of low-frequency current pulsesbetween the first current level and the second current level and back tothe first current level for writing servo marks; the closing phasecomprises a closing pattern of high-frequency current pulses between thefirst current level and the second current level and back to the firstcurrent level; and wherein servo marks are written only during the servomark write phase.

These and other aspects of the invention are described below.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the invention are set forth in the appended claims. Theinvention itself, however, will be best understood by reference to thefollowing detailed description of illustrative embodiments when read inconjunction with the accompanying drawings, wherein:

FIG. 1A is a cutaway cross-section view through line 1A-1A of FIG. 1Billustrating a servo write head according to an embodiment of thepresent invention;

FIG. 1B is a bottom view of the servo write head of FIG. 1A;

FIG. 1C is a top view of servo mark positions on magnetic tape;

FIG. 2A is plot of write current levels to be applied to servo writeheads versus medium position according to first embodiments of thepresent invention;

FIG. 2B is a sectional view of the magnetization state of the mediumplotted as medium depth versus medium position generated by the writesignal of FIG. 2A;

FIG. 2C is a plot of the readback signal of medium written with thewrite signal of FIG. 2A;

FIGS. 3A and 3B are plots of write current levels to be applied to servowrite heads versus medium position according to alternative firstembodiments of the present invention;

FIG. 4A is a plot of write current levels to be applied to servo writeheads versus medium position according to second embodiments of thepresent invention;

FIG. 4B is a section view of the magnetization state of the mediumplotted as medium depth versus medium position generated by the writesignal of FIG. 4A;

FIG. 4C is a plot of the readback signal of medium written with thewrite signal of FIG. 4A;

FIG. 5 is a plot of pulse amplitude of the undesired servo mark signalsvs. position on the magnetic tape;

FIG. 6 is a plot of the ratio of the amplitude of the undesired signalto amplitude of the servo mark signal vs. number of pulses in thehigh-frequency preamble;

FIG. 7 is a flowchart of the method of writing servo marks according tofirst embodiments of the present invention; and

FIG. 8 is a flowchart of the method of writing servo marks according tosecond embodiments of the present invention.

DETAILED DESCRIPTION

The present invention is directed to new methods for producing highoutput timing-based magnetic servo patterns on an AC erased magneticstorage media. The methods utilize preambles before servo write andclosings after servo write. The preambles before servo write create atransition from an AC erased media to a DC erased media immediatelybefore the servo marks. The closings after servo write create atransition from a DC erased media back to an AC erased media immediatelyafter the servo marks. In first embodiments, the preambles are abruptcurrent changes prior to current ramps and the closings are currentramps followed by abrupt current changes. This method improvesconsiderably the efficiency in reducing undesired readback pulses bytailoring the write current ramp more effectively. In secondembodiments, both the preambles and closings are sets of high-frequencycurrent pulses. The second embodiments achieve high output timing-basedservo signal on AC erased media without the need to control a ramp ofcurrent. The transition from AC erase media to DC erase media before andafter the servo marks is made using properly designed high-frequencywrite current signals.

FIG. 1A is a cutaway cross-section view through line 1A-1A of FIG. 1Billustrating a servo write head according to an embodiment of thepresent invention. In FIG. 1A, a dual-gap servo write head 100 includesferromagnetic body 105 having a left gap 110L and a right gap 110Rspaced a center-to-center distance Sg apart and an induction coil 115.Ferromagnetic body 105 need not be formed from iron but has the propertyof being ferromagnetic. Left and right gaps 110L and 110R haverespective widths Wl and Wr. Induction coil 115 is electricallyconnected to a bipolar DC servo write driver 120 which generates avarying current signal that is applied to induction coil 115. Oppositeends of coil 115 are electrically connected to respective positive andnegative current terminals of bipolar servo write driver 120. A magnetictape 125 having a thickness T is spaced a distance S (in theZ-direction) under dual-gap servo write head 100 and is moving at avelocity V in the X-direction. When a current i(t) is applied to coil115, a magnetic write bubble 130L is induced in magnetic tape 125 undergap 110L which magnetizes a region 135L of the magnetic tape. The samecurrent i(t) induces a magnetic write bubble 130R in magnetic tape 125under gap 110R which magnetizes a region 135R of the magnetic tape. Themagnetic regions 135L and 135R are wider (in the X-direction) than thewrite bubbles because magnetic tape 125 is moving from right to leftwhile the write current i(t) remains at a constant positive value inthis example.

Regions 135R and 135L will be magnetized when the write field strengthsin the X-direction (Hx) generated by the gaps 110L and 110R are greaterthan the coercivity of the magnetic medium (Hc). Each write bubble 130Land 130R has two edges. The leading edge is the rightmost edge and thetrailing edge is the leftmost edge. The average width (in theX-direction) of write bubbles 130L and 130R depends on the distance S,the amplitude of the write current, the widths Wl and Wr, and thecoercivity of magnetic tape 125. For optimum writing, the widths ofwrite bubbles 130L and 130R should be about the same as the widths ofgaps 110L and 110R respectively. Wl and Wr may be the same or may bedifferent. As can be seen, both regions 135L and 135R are written at thesame time. The widths (in the X-direction) of regions 135L and 135R area function of the velocity V of magnetic tape 125, the time duration ofthe current i(t), and the write bubble parameters discussed supra.

FIG. 1B is a bottom view of the servo write head of FIG. 1A. In FIG. 1B,gaps 110L and 110R are trapezoidal in shape and are tilted in theY-direction by respective angles Al and Ar. Gaps 110L and 110R aretilted toward each other. The X, Y and Z directions are mutuallyorthogonal. The magnitudes of angles Al and Ar may be the same ordifferent.

Although magnetic tape 125 is depicted as under servo write heads 100and 140, alternatively, FIG. 1A may be rotated 180° about the Y-axis sothe magnetic tape passes over the servo write heads in which case FIG.1B would depict the top surface of the servo write heads.

FIG. 1C is a top view of servo mark positions on a magnetic tape. InFIG. 1C, only one servo track is depicted. There may be multiple servotracks on the same tape parallel to each other. Magnetic tape 125 has aservo mark centerline 140 and two sets of servo marks; an “AB” set 145ABand a “CD” set 145CD. Set 145AB includes two servo marks 150A written bygap 110L (of FIG. 1A) and two servo-marks 150B written by gap 110R (ofFIG. 1A). Set 145CD includes two servo marks 150C written by gap 110L(of FIG. 1A) and two servo marks 150D written by gap 110R (of FIG. 1A).FIG. 1C also shows the start of a following “AB” servo mark 145AB(2)having two servo marks 150E written by gap 110R (of FIG. 1A). Set 145ABis written by applying two write pulses to servo write head 100. Set145CD is written by applying two write pulses to servo write head 100after set 145AB has been written. Set 145AB(2) is written by applyingtwo write pulses to servo write head 100 after set 145CD has beenwritten. “AB” and “CD” servo marks will alternate along the length ofmagnetic tape 125. The servo track has a width Sh and several distancesalong the length of the tape (X-direction) are defined. The distancefrom the left (right) edge (all measurement are along centerline 140) ofthe leftmost servo mark 150A to the left (right) edge of leftmost servomark 150B is AB. The distance AB is equal to the gap-to-gap distance Sgof FIG. 1A. The distance from the left (right) edge of the leftmostservo mark 150C to the left (right) edge of leftmost servo mark 150D isCD. The distance CD is equal to the gap-to-gap distance Sg of FIG. 1A.The distance from the left (right) edge of the leftmost servo mark 150Ato the left (right) edge of leftmost servo mark 150C is AC. The distancefrom the left (right) edge of the leftmost servo mark 150C to the left(right) edge of leftmost servo mark 150E is CA. While pairs of servomarks 150A, 150B, 150C and 150D (and 150E) are illustrated, there may beone or more servo marks 150A, 150B, 150C and 150D (and 150E). Also thenumber of servo marks in the AB servo mark pairs may be different fromthe number of servo marks in the CD servo mark pairs.

FIG. 2A is plot of write current levels to be applied to servo writeheads versus medium position according to first embodiments of thepresent invention. Magnetic tape passing the servo write head ismagnetized when the applied current has sufficient amplitude (eitherpositive or negative). In the present illustration positive currentwrites the magnetic medium in the +X-direction and is used for writingservo marks. Negative current writes the magnetic medium in the−X-direction and is used for DC-erasing the magnetic medium before andafter each servo mark as well as for transitioning from the AC erasedstate to the DC erased state before writing the servo marks and fortransitioning from the DC erased state to the AC erased state afterwriting the servo marks. Alternatively, positive currents may be used toerase and negative currents to write.

Returning to FIG. 2A, the current waveform 155 is generated, forexample, by bipolar servo write driver 120 of FIG. 1A. In FIG. 2A, d0,d1, d2, d3, d4, d5, d6 and d7 are positions along the magnetic tape thatdefine sequential segments of the tape. The positions can also bethought as defining the lengths of the segments or distances betweenpositions along the tape. Between positions d0 and d1 a current i0 isapplied. Current i0 is either zero current, or a near zero current (anear zero current is defined as a positive or negative current that doesnot change the magnetic state of the medium). At d1 the current abruptlytransitions to i1. An abrupt change in current is defined as a ratechange in current (dI/dt) primarily limited by the rise or fall time ofthe write driver and can be as short as a few (e.g., 5) nano-seconds(ns). At a tape velocity (V) of 5 meters/second (m/s), 5 ns correspondsto a distance of 0.025 μm. Between d1 and d2, the currents ramps from i1to i2. At d2, the current abruptly transitions to i4. In the example ofFIG. 2A, current i4 is positive enough to write the servo track anddefine the left edge of the servo mark. Between d2 and d3, the currentremains at i4. At d3 the current abruptly transitions to i3. Between d3and d4 the current remains at i3. In the example of FIG. 2A, current i3is negative enough to define the right edge of the servo mark as well asDC erase the servo track. At d4 the current abruptly transitions to i4and defines the left edge of the second servo mark. Between d4 and d5,the current remains at i4. At distance d5 the current abruptlytransitions to i2 and defines the right edge of the second servo mark.In FIG. 2A, N=2 servo marks are written.

The number N of servo marks can be any integer equal to or greater thanone. To write N servo marks, N positive write pulses occur between d2and d5. Each write pulse corresponds to the write current abruptlytransitioning from a negative current (of amplitude i2 or i3) to apositive current (of amplitude i4) to a negative current (of amplitudei2 or i3). Between d5 and d6, the currents ramps from i2 to i1. At d6,the current abruptly transitions to i0. The current remains at i0between d6 and d7 (between d7 and d1 AB servo marks are written; d7 isequivalent to d1 when writing CD servo marks). The distance between d6and d7 prevents overwriting of the AB servo marks with the CD servomarks. Between d1 and d2 and between d5 and d6 the medium is onlypartially written through its depth. The segments between d1 and d2 andbetween d5 and d6 correspond respectively to the transition fromAC-erased medium to DC-erased medium before the servo marks and fromDC-erased medium to AC erased medium after the servo marks. Servo marksare written only in the segments between d2 and d5. In the example ofFIG. 2A, i4>(i0≈0)>i1>i2>i3. Alternatively, i4<(i0≈0)<i1<i2<i3. In oneexample, |i4|=|i3|.

The pattern of write current described in FIG. 2A repeats to define theCD servo marks. If servo marks of the CD pairs are identical to theservo marks of the AB pairs (in number, width and spacing), then thedistances d7-d6 (or d1-d0), d6-d5, d5-d4, d4-d3, d3-d2, d2-d1 don't needto be changed. In the more general case, the number of servo marks inthe CD pairs and in the AB pairs may be different and the distancesd7-d6, d6-d5, d5-d4, d4-d3, d3-d2, d2-d1 are adjusted for writing eachpair of servo marks: (d7-d6)_(AB), (d6-d5)_(AB), (d5-d4)_(AB),(d4-d3)_(AB), (d3-d2)_(AB), (d2-d1)_(AB) for writing the AB pairs and(d7-d6)_(CD), (d6-d5)_(CD), (d5-d4)_(CD), (d4-d3)_(CD), (d3-d2)_(CD),(d2-d1)_(CD) for writing the CD pairs. If the number of servo marks inthe AB servo mark pairs is N1 and the number of servo marks in the CDservo mark pairs is N2, then N1 positive pulses are applied during(d5-d2)_(AB) to write the AB marks and N2 positive pulses are appliedduring (d5-d2)_(CD) to write the CD servo marks. Both N1 and N2 arepositive integers greater than one. If the number of AB and CD servomarks are the same then N1 equals N2. If the number of AB and CD servomarks are not the same then N1 does not equal N2.

The X-scale of FIG. 2A may be converted from a medium position scale toa time scale by dividing the position scale by the velocity V of themagnetic tape past the servo write head. When write current is plottedversus the position of the moving magnetic medium positions d0, d1, d2,d3, d4, d5 and d6 are used. By dividing by V, the distance d1-d0 becomesa time interval t1, d2-d1 becomes a time interval t2, d5-d2 becomes atime interval t3, d6-d5 becomes a time interval t4; and the distanced7-d6 becomes a time interval t5.

In terms of tape position, in the distance d2-d5 the current alternates(in pulses of controlled time duration) from negative to positive tonegative two times (in this example) to write a pair of A and B servomarks (or C and D servo marks). Note that the width of the servo marksis defined by the duration of the positive pulses and is independent ofthe width of the write gap. Similarly, the distance between two marks isdefined by the duration of the negative current between two consecutivepositive current pulses and is independent of the width of the writegap. Moreover, the servo mark edges are defined by the trailing edge ofthe write gap only. In terms of time, during time t3 the currentalternates from negative to positive to negative two times (in thisexample) to write a pair of A and B servo marks (or C and D servomarks). The same sequence can be repeated for additional A and B servomark pairs and additional C and D servo marks pairs.

The inequalities of Table I set a limitation for the duration of thecurrent ramps. Using the notation of the general case, for AB pulsesd1ab=(d2-d1)_(AB), d2ab=(d5-d2)_(AB), d3ab=(d6-d5)_(AB), andd4ab=(d7-d6)_(AB). For CD pulses d1cd=(d2−d1)_(CD), d2cd=(d5-d2)_(CD),d3cd=(d6-d5)_(CD) and d4cd=(d7-d6)_(CD) which gives:

AC=d2ab+d3ab+d4ab+d1cd  (1)

CA=d2cd+d3cd+d4cd+d1ab  (2)

The current ramps are d1ab, d3ab, d1cd and d3cd. Wl, Wr and Sg areillustrated in FIG. 1A. Angles Al and Ar are illustrated in FIG. 1B.Distances AB=CD=Sg, AC, CA and servo track width Sh are illustrated inFIG. 1C.

TABLE I In order that: The following condition must be satisfied: 1There be no overwrite of d3ab < Sg − d2ab − Wl − [(Sh/2) * B marks afterd3ab ((tan(Al) + tan(Ar))] 2 There be no overwrite of d1cd < AC − Sg −d2ab − [(Sh/2) * B marks after d4ab ((tan(Al) + tan(Ar))] 3 There be nooverwrite of d3cd < Sg − d2cd − Wl − [(Sh/2) * D marks after d3cd((tan(Al) + tan(Ar))] 4 There be no overwrite of d1ab < CA − Sg − d2cd −[(Sh/2) * D marks after d4cd ((tan(Al) + tan(Ar))]

Where:

Sg is the center-to-center distance between the left and right gaps (seeFIG. 1A);

Wl is the width of the left gap in the X-direction;

Wr is the width of the right gap in the X-direction;

Al is the angle of the left gap slanted away from the Y-direction;

Ar is the angle of the right gap slanted away from the Y-direction;

d1ab, d2ab, d3ab, d4ab, d1cd, d2cd, d3cd and d4cd are sequentialsegments along the magnetic tape in the X-direction where:

-   -   in segment d1ab the current ramps negative;    -   in segment d2ab the current pulses from negative to positive and        back to negative N1 times, where N1 is a positive integer equal        to or greater than one;    -   in segment d3ab the current ramps positive;    -   in segment d4ab the current is zero or near zero;    -   in segment d1cd the current ramps negative;    -   in segment d2cd the current pulses from negative to positive and        back to negative N2 times, where N2 is a positive integer equal        to or greater than one;    -   in segment d3cd the current ramps positive;    -   in segment d4cd the current is zero; and    -   Sh is the width of the servo track in the Y-direction; and

the X-direction is defined as the direction of movement of the magneticstorage medium (e.g., magnetic tape) from the right gap to the left gapand the Y-direction is defined as a direction in the plane of the mediumperpendicular to the X-direction.

Note that the number N1 of servo marks in the AB servo mark pairs may bethe same as the number of server mark N2 in the CD servo mark pairs(N1=N2), or the number N1 of servo marks in the AB servo mark pairs maybe different from the number of server mark N2 in the CD servo markpairs (N1≠N2).

FIG. 2B is a sectional view of the magnetization state of the mediumplotted as medium depth versus medium position generated by the writesignal of FIG. 2A. The section is along the Y-direction for a region ofthe medium. In FIG. 2B, the regions labeled “L” are written with themagnetization in the −X (left) direction, and the regions labeled “R”are written with the magnetization in the +X (right) direction. and theregions labeled “AC” remain AC-erased.

FIG. 2C is a plot of the readback signal of medium written with thewrite signal of FIG. 2A. In FIG. 2C waveform 160 represents the readbacksignal. It is instructive to compare regions 161 of waveform 160 todashed waveforms 162. Regions 161 are regions of undesired signal.Ideally, the signal would be zero in regions 161. Dashed waveforms 162represent the undesired signal that would be present without the abrupttransitions of current from i0 to i1 at d1 and from i2 to i0 at d6 onwaveform 155 of FIG. 2A (i.e., with a ramp from i0 at d1 to i2 at d2 anda ramp of i2 at d5 to i0 at d6). As can be seen the abrupt change incurrent at X-positions d1 and d2 significantly reduces the amplitude ofthe undesired signal when compared to a ramp only.

The amplitude of the undesired signal in regions 161 is directly relatedto the extension of the magnetization transition from fully AC erased(across the depth of the medium) to fully DC erased (across the depth ofthe medium). The more abrupt this magnetization transition, the largerthe undesired signal in 161. Inversely, the more extended the transitionof the medium magnetization from fully AC erased to fully DC erased, thesmaller the undesired signal in 161. As depicted in the FIG. 2B, a slowramp in write current from i1 to i2 does result in an extendedtransition of the medium magnetization from fully AC erased to fully DCerased. Similarly, a slow ramp in write current from i2 to i1 doesresult in an extended transition of the medium magnetization from fullyDC erased to fully AC erased. The longer the ramp in current, thesmaller the undesired signal in regions 161.

However, there is a limit to the extent of the current ramp for a giventiming based servo pattern as discussed supra. In Linear Tape Open (LTO)technology tape drives with d2ab=17 μm, d3cd=22 μm, AB=50 μm, AC=CA=100μm, Al=Ar=6°, Sh=186 μm and further assuming that d1ab=d3ab=d1cd=d3cd,the maximum ramp length is about 7.4 μm.

To optimize the ramp of the write current, current levels i1 and i2 needto be selected appropriately. Below a certain value of write current,the write head does not produce fields that are large enough to changethe magnetization state of the medium. This minimum current valuerequired to write in the medium depends on the head medium spacing S,the write gap dimensions and the medium coercivity. Above a certainvalue of write current, the write head produces fields that are largeenough to write the full depth of the medium, but still lower than thecurrent needed to write optimum straight magnetization transitions(current levels i3 and i4). In FIG. 2A, the current i1 is required to becomparable to the minimum current value required to change themagnetization in the medium and i2 is required to be comparable to thecurrent just required to write fully through the medium. The length oftape over which the ramp is applied is limited in length and should beshort as discussed supra. With ramps starting at i0 and ending at i3,the length of tape over which the ramp is applied would generally belarge and therefore there would be an increase in the amplitude of theundesired signal compared to the optimized ramps lengths of the presentinvention. The optimized ramps of the present invention have an abruptchange of current from i0 to i1 at d1 and from i1 to i0 at d6. The writecurrent is ramped from i1 to i2 between d1 and d2 and ramped from i2 toi1 between d5 and d6. Using this method and for a ramp of currentspreading over 5 μm of the medium (d2−d1=5 μm), the amplitude of theundesired pulses in a thick medium (e.g., 100 nm) can be reduced to lessthan 10% of the amplitude of the timing based servo signal (at d3 andd4). This result is based on a numerical calculation with a write gap of500 nm, a head-medium spacing of 30 nm, a servo reader of width of 3.5μm and of 310 nm read gap. Comparatively, using a slope from i0 to i3(and from i3 to i0) leads to undesired peaks in regions 161 of about 35%of the amplitude of the timing based servo signal. It can also be shownthat the abrupt ramp method is also efficient for a thin medium (e.g.,50 nm), whereas methods using a ramp starting at i0 were not, which isan unexpected benefit. In a thin medium the amplitude of the undesiredpulses was reduced to less than 20% of the amplitude of the timing basedservo signal.

FIGS. 3A and 3B are plots of write current levels to be applied to servowrite heads versus medium position according to an alternative firstembodiment of the present invention. Waveform 165 of FIG. 3A is similarto waveform 155 of FIG. 2A, except between d1 and d2 the current rampsfrom i1 to i3 instead of to i2, and between d5 and d6 the current rampsfrom i3 to i1 instead of from i2. In FIG. 3A, i4>(i0≈0)>i1>i3.Alternatively, i4<(i0≈0)<i1<i3. In FIG. 3A, the ramps extend to the samecurrent level as between the pulses. Waveform 170 of FIG. 3B is similarto waveform 155 of FIG. 2A except between d1 and d1.1 the current rampsfrom i1 to i2, from d1.1 to d2 the current abruptly decreases to i3 andremains at i3 until d2 where the current abruptly transitions to i4, atd5 the current abruptly decreases to i3 where it remains until d5.1where it abruptly transitions to i2, and between d5.1 and d6 the currentramps from i2 to i1. In FIG. 3B, i4>(i0≈0)>i1>i2>i3. Alternatively,i4<(i0≈0)<i1<i2<i3. In FIG. 3B, an abrupt change to the current levelbetween pulses intervenes between the ramps and the current pulses.

In second embodiments of the present invention, patterns ofhigh-frequency write currents are used in the preamble and closing. Theinequalities of Table I, discussed infra, set a limitation for theduration of the high-frequency preamble and closing.

FIG. 4A is a plot of write current levels to be applied to servo writeheads versus medium position according to second embodiments of thepresent invention. Magnetic tape passing the servo write head ismagnetized when the applied current has sufficient amplitude (eitherpositive or negative). In the present illustration positive currentwrites the magnetic medium in the +X-direction and is used for writingservo marks and during the high frequency preamble and closing. Negativecurrent writes the magnetic medium in the −X-direction and is used forDC-erasing the magnetic medium on either side of each servo mark andduring the high frequency preamble and closing. Alternatively, positivecurrents may be used to erase and negative currents to write.

In FIG. 4A, a current waveform 180 is generated, for example, by bipolarservo write driver 120 of FIG. 1A. In FIG. 4A, do, d1, d1.5, d2, d3, d4,d5, d5.5, d6 and d7 are positions along the magnetic tape that definesequential segments of the tape. The positions can also be thought asdefining the lengths of the segments or distances between positionsalong the tape. Between d0 and d1 a current i0 is applied. Again,current i0 is either zero current, or a near zero current. At distanced1 the current abruptly drops to i3. Between d1 and d1.5, a pattern of Mhigh-frequency current pulses varying from i3 to i4 and back to i3 areapplied. In FIG. 4A, M=2, but M may be any integer equal to or greaterthan one. Between d1.5 and d2 the current remains at i3. There is aminimum value for d2-d1.5 called D_(min). D_(min) is about equal to thewidth of the read gap of the read head to be used to read the servomarks. At d2, the current abruptly transitions to i4. In the example ofFIG. 4A, current i4 is positive enough to write the servo track anddefine the left edge of each servo mark. Between d2 and d3, the currentremains at i4. At d3 the current abruptly transitions to i3. Between d3and d4 the current remains at i3. In the example of FIG. 4A, current i3is negative enough to define the right edge of each servo mark and DCerase the servo track immediately before and after each servo mark. Atd4 the current abruptly transitions to i4 and defines the left edge ofthe second servo mark. Between d4 and d5, the current remains at i4. Atd5 the current abruptly transitions to i3 and defines the right edge ofthe second servo mark. Between d2 and d5 there are N servo low-frequencywrite pulses. In one example, the pulse widths and distances betweenpulses for the patterns of high-frequency current pulses (between d1 andd1.5 or between d5.5 and d6) are 10 to 200 times smaller than the pulsewidths and distances between the pulses for the low-frequency writepulses (between d2 and d5). In FIG. 4A, N=2, but N can be any integerequal to or greater than one. To write N servo marks, N positive writepulses take place between d2 and d5. Each low-frequency write pulsecorresponds to the write current abruptly transitioning from negativecurrent (e.g., i3) to positive current (e.g., i4) to negative current(e.g., i3). Between d5 and d5.5 the current remains at i3. Between d5.5and d6, a pattern of P high-frequency pulses from i3 to i4 and back toi3 are applied. In FIG. 4A, P=2, but P may be any integer equal to orgreater than one. P may be the same or different from M. The value ofd5.5-d5 is greater than or equal to D_(min). At d6 the current returnsto i0. The current remains at i0 for a distance d7-d6 (if d6-d0 writesAB servo marks, then d7 is equivalent to d1 for writing CD servo marks).The distance d7-d6 prevents overwriting of the AB servo marks by the CDservo marks. In the distance d1.5-d1 and d6-d5.5 magnetizationtransitions are written in the medium, but they are positioned so closeto each other that their contributions to the readback signal is verysmall, if any. Servo marks are written only in the distance d5-d2.

Because the last written magnetic domain has the size of thewrite-bubble, the pattern of high-frequency current pulses of theclosing should be slightly different from the pattern of high-frequencycurrent pulses of the preamble in that the width of and distancesbetween high-frequency pulses in the closing (cw1, cs1, cw2, cs2) aredifferent than in the preamble (ps1, pw1, ps2, pw2). It is preferredthat pw1≠pw2 and ps1≠ps2, or pw1=pw2 and ps1≠ps2, or pw1≠pw2 andps1=ps2. It is preferred that cw1≠cw2 and cs1≠cs2, or cw1=cw2 andcs1≠cs2, or cw1≠cw2 and cs1=cs2. In other words, it is preferred thatthe preamble and closing not consist of periodic pulses (i.e., notconsist of a repeating pattern of same width pulses spaced the samedistance apart) but be patterns of high frequency pulses of controlledand varying widths and spaces. The width of the lower-frequency writepulses is w3 and the space between pulses is s3. w3>>w1 or w2 and s3>>s1or s2.

The X-scale of FIG. 4A may be converted from a medium position scale toa time scale by dividing the position scale by the velocity V of themagnetic tape past the servo write head. When write current is plottedversus the position of the moving magnetic medium distances, d0, d1,d1.5, d2, d3, d4, d5, d5.5 and d6 are used. By dividing by V, thedistance d1-d0 becomes a time interval t1, d1.5-d1 becomes a timeinterval t2, d2-d1.5 becomes a time interval t3, d5-d2 becomes a timeinterval t4, d6-d5.5 becomes a time interval t5, d6-d5.5 becomes a timeinterval t6; and the distance d7-d6 becomes a time interval t7. In FIG.2A, i4>(i0≈0)>i3. Alternatively, i4<(i0≈0)<i3. It is preferred that|i4|=|i3|, however, in one example, |i4|≠|i3|.

The pattern of write current described in FIG. 4A repeats to define theCD servo marks. If servo marks of the CD pairs are identical to theservo marks of the AB pairs (in number, width and spacing), then thedistances d7-d6 (or d1-d0), d6-d5, d5-d4, d4-d3, d3-d2, d2-d1 don't needto be changed. In the more general case, the number of servo marks inthe CD pairs and in the AB pairs may be different, and the distancesd7-d6, d6-d5, d5-d4, d4-d3, d3-d2, d2-d1 are adjusted for writing eachpair of servo marks: (d7-d6)_(AB), (d6-d5)_(AB), (d5-d4)_(AB),(d4-d3)_(AB), (d3-d2)_(AB), (d2-d1)_(AB) for writing the AB pairs and(d7-d6)_(CD), (d6-d5)_(CD), (d5-d4)_(CD), (d4-d3)_(CD), (d3-d2)_(CD),(d2-d1)_(CD) for writing the CD pairs. If the number of servo marks inthe AB servo mark pairs is N1 and the number of servo marks in the CDservo mark pairs is N2, then N1 positive pulses are applied during(d5-d2)_(AB) to write the AB marks and N2 positive pulses are appliedduring (d5-d2)_(CD) to write the CD servo marks. Both N1 and N2 arepositive integers greater than one. If the number of AB and CD servomarks are the same then N1 equals N2. If the number of AB and CD servomarks are not the same then N1 does not equal N2.

In the general case, the number and pattern of M pulses and P pulsesused in the preamble and closing may be the same or different for AB andCD servo mark pairs, regardless of whether or not the number of servomarks in the AB pairs are the same as in the CD pairs. AB servo markpairs may use a pattern of M1 and a pattern of P1 pulses, and CD pairsmay use a pattern of M2 pulses and a pattern of P2 pulses. It ispreferred that the patterns of P1, P2, M1 and M2 pulses not consist ofperiodic pulses (i.e., not consist of a repeating pattern of same widthpulses spaced the same distance apart) but be patterns of high frequencypulses of controlled and varying widths and spaces. M1, M2, P1 and P2are positive integers greater than one. This yields multiplepermutations, ten of which are: (1) N1=N2, M1=M2=P1=P2; (2) N1=N2,M1=M2, P1=P2, M1≠P1; (3) N1=N2, M1≠M2, P1=P2; (4) N1=N2, M1=M2, P1≠P2;(5) N1=N2, M1≠M2, P1≠P2; (6) N1≠N2, M1=M2=P1=P2; (7) N1≠N2, M1=M2,P1=P2, M1≠P1; (8) N1≠N2, M1≠M2, P1=P2; (9) N1≠N2, M1=M2, P1≠P2; (10)N1≠N2, M1≠M2, P1≠P2.

In terms of tape position, in between d2-d5 the current alternates (inpulses of controlled time duration) from negative to positive tonegative two times (in this example) to write a pair of A and B servomarks (or C and D servo marks). Note that the width of the servo marksis defined by the duration of the positive pulses and is independent ofthe width of the write gap. Similarly, the distance between two marks isdefined by the duration of the negative pulses and is independent of thewidth of the write gap. Moreover, the servo mark edges are defined bythe trailing edge of the write gap only. In terms of time, during timet4 the current alternates from negative to positive to negative topositive to negative to write a pair of A and B servo marks (or C and Dservo marks). The same sequence can be repeated for additional A and Bservo mark pairs and additional C and D servo marks pairs.

The inequalities of Table I set a limitation for the duration of thepattern of high-frequency current pulses. Using the notation of thegeneral case, for AB pulses d1ab=(d2-d1)_(AB), d2ab=(d5-d2)_(AB),d3ab=(d6-d5)_(AB), and d4ab=(d7-d6)_(AB). For CD pulsesd1cd=(d2−d1)_(CD), d2cd=(d5-d2)_(CD), d3cd=(d6-d5)_(CD) andd4cd=(d7-d6)_(CD) which gives:

AC=d2ab+d3ab+d4ab+d1cd  (1)

CA=d2cd+d3cd+d4cd+d1ab  (2)

The patterns of high-frequency current pulses occur in d1ab, d3ab, d1cdand d3cd. Wl, Wr and Sg are illustrated in FIG. 1A. Angles Al and Ar areillustrated in FIG. 1B. Distances AB=CD=Sg, AC, CA and servo track widthSh are illustrated in FIG. 1C,

in which:

-   -   Sg is the center-to-center distance between the left and right        gaps;    -   Wl is the width of the left gap in the X-direction;    -   Al is the angle of the left gap slanted away from the        Y-direction;    -   Ar is the angle of the right gap slanted away from the        Y-direction;    -   d1ab, d2ab, d3ab, d4ab, d1cd, d2cd, d3cd and d4cd are sequential        segments along the magnetic tape in the X-direction where:        -   in segment d1ab the current varies from negative to positive            to negative M1 times following a first preamble pattern (a            non-periodic pattern is preferred) of M1 high frequency            pulses of controlled widths and spacing, where M1 is a            positive integer equal to or greater than one;        -   in segment d2ab the current pulses from negative to positive            and back to negative N1 times, following a first servo mark            pattern (in one example, when used for timing and tracking,            a periodic pattern; however, when servo marks are used to            encode information the pattern may be non-periodic) of N1            low frequency pulses of controlled widths and spacings,            where N1 is a positive integer equal to or greater than one;        -   in segment d3ab the current varies from negative to positive            to negative P1 times following a first closing pattern (a            non-periodic pattern is preferred) of P1 high frequency            pulses of controlled widths and spacing, where P1 is a            positive integer equal to or greater than one;        -   in segment d4ab the current is zero or near zero;        -   in segment d1cd the current varies from negative to positive            to negative M2 times, following a second preamble pattern (a            non-periodic pattern is preferred) of M2 high frequency            pulses of controlled widths and spacing, where M2 is a            positive integer greater than or equal to one;        -   in segment d2cd the current pulses from negative to positive            and back to negative N2 times, following a second servo mark            pattern (in one example, when used for timing and tracking,            a periodic pattern; however, when servo marks are used to            encode information the pattern may be non-periodic) of N2            low frequency pulses of controlled widths and spacings,            where N2 is a positive integer greater than or equal to one;        -   in segment d3cd the current varies from negative to positive            to negative P2 times, following a second closing pattern (a            non-periodic pattern is preferred) of P2 high frequency            pulses of controlled widths and spacing, whereP2 is a            positive integer greater than or equal to one;        -   in segment d4cd the current is zero or near zero;        -   the pulse widths and distances between pulses for the            patterns of high-frequency current pulses are much smaller            than the pulse widths and distances between the pulses for            the patterns of low-frequency current pulses; and    -   Sh is the width of the servo track in the Y-direction; and    -   the X-direction is defined as the direction of movement of the        magnetic storage medium (e.g., magnetic tape) from the right gap        to the left gap and the Y-direction is defined as a direction in        the plane of the medium perpendicular to the X-direction.

FIG. 4B is side view of the magnetization state of the medium plotted asmedium depth versus medium position generated by the write signal ofFIG. 4A. The section is along the Y-direction for a region of themedium. In FIG. 4B, the clear regions are written with the magnetizationin the −X (left) direction, the cross-hatched regions are written withthe magnetization in the +X (right) direction, and the clear regionslabeled “AC” remain AC-erased. The clear regions labeled “L” are DCerased regions between servo marks.

FIG. 4C is a plot of the readback signal of medium written with thewrite signal of FIG. 4A. In FIG. 4C waveform 185 represents the readbacksignal. Regions 186 and 187 are regions of undesired signal. Ideally,the signal would be zero in regions 186 and 187. Region 186 is discussedinfra, with respect to FIG. 5.

FIG. 5 is a plot of pulse amplitude of the undesired servo mark signalsvs. position on the magnetic tape. Waveform 190A is the region 186 ofwaveform 185 of FIG. 4C that would be obtained without a high-frequencypreamble, and waveform 190B is the region 186 of waveform 185 of FIG. 4Cthat is obtained with pattern of M=7 high-frequency pulses in thepreamble. Defining PW50 as the width of a pulse resulting from a stepfunction of current varying from i3 to i4, the pattern of M=7 pulsescorresponds to ps1=1.5*PW50, pw1=0.15*PW50, ps2=0.35*PW50, pw2−0.2*PW50,ps3=0.35*PW50, pw3=0.15*PW50, ps4=0.4PW50, pw4=0.15*PW50, ps5=0.4*PW50,pw5=0.1*PW50, ps6=0.5*PW50, pw6=0.1*PW50, ps7=0.5*PW50, pw7=0.05*PW50.As can be seen the high-frequency preamble current significantly reducesthe amplitude of the undesired signal by about 70%. Similar results areobtained in region 187 of waveform 185 of FIG. 4C using thehigh-frequency closing. This is an unexpected result.

FIG. 6 is a plot of the ratio of the amplitude of the undesired signalto amplitude of the servo mark signal vs. number of pulses in thehigh-frequency preamble. In FIG. 6, curve 195 illustrates thatincreasing the number of pulses in the high-frequency preamble allows acontinuous decrease the amplitude of the undesired signal.

FIG. 7 is a flowchart of the method of writing servo marks according tofirst embodiments of the present invention. In the followingdescription, except when referring to numbers of pulses, “negative” maybe substituted for “positive” and “positive” substituted for “negative.”

In step 200, the servo write signal (e.g., 155 of FIG. 3A, 165 of FIG.3A, 170 of FIG. 3B) is designed to the conditions of TABLE I. The use ofa general purpose computer as an aid in the design of the servo writesignal is useful. The servo write signal is supplied to the servo writehead (e.g., dual-gap servo write head 100 of FIG. 1A). During steps 205through 230, the magnetic tape is moving at a constant velocity past theservo write head. Signal design techniques include graphical and numericmethods.

In step 205, from a zero or near zero current level i0, a negativecurrent level i1 is applied to the servo write head at a time T1.Alternatively, when the number of AB servo marks is to be different fromthe number of CD servo marks, the negative current level i1 is appliedfor a time T1(1) or T1(2) on alternating passes through the loop 205,210, 215, 220, 225 and 230.

In step 210, the current is ramped further negative to a negativecurrent level i2 until a time T2. Alternatively, the current is rampedfurther negative to negative current level i2 until time T2.1<T2, thenset to i3 from time T2.1 to time T2, where i3 is more negative than i2.Alternatively, the current is ramped further negative to negativecurrent level i3 until time T2.

Alternatively, in step 210, when the number of AB servo marks is to bedifferent from the number of CD servo marks, the current is rampedfurther negative to a negative current level i2 until a time T2(1) orT2(2) on alternating passes through the loop 205, 210, 215, 220, 225 and230. Alternatively, the current is ramped further negative to negativecurrent level i2 until time T2.1(1)<T2(1) or T2.1(2)<T2(2) then set toi3 from time T2.1(1) or T2.1(2) to time T2(1) or T2(2), where i3 is morenegative than i2 on alternating passes through the loop 205, 210, 215,220, 225 and 230. Alternatively, the current is ramped further negativeto negative current level i3 until time T2(1) or T2(2) on alternatingpasses through the loop 205, 210, 215, 220, 225 and 230.

In step 215, the current is pulsed N times from negative current leveli3 to positive current level i4 and back to negative current level i3until a time T3. In one example |i3|=|i4|. In one example |i3|≠|i4|. Nis a positive integer equal to or greater than one. During time durationT3-T2, pairs of N servo mark are written to the servo track across fromboth gaps. The trailing edge of the last pulse ends at current level i2.

Alternatively, in step 215, when the number of AB servo marks is to bedifferent from the number of CD servo marks, the current is pulsed N1 orN2 times from negative current level i3 to positive current level i4 andback to negative current level i3 until a time T3(1) or T3(2). In oneexample |i3|=|i4|. In one example |i3|≠|i4|. N1 and N2 are positiveintegers equal to or greater than one, and N1 is not equal to N2. Duringtime duration T3(1)-T2(1), N1 pairs of servo mark are written to theservo track across from both gaps. During time duration T3(2)-T2(2), N2pairs of servo mark are written to the servo track across from bothgaps. The trailing edge of the last pulse ends at current level i2. N1pulses or N2 pulses are applied on alternating passes through the loop205,210, 215, 220, 225 and 230.

In step 220, between time T3 and a time T4, the current is ramped tocurrent level i1. Alternatively, instead of ramping the current startingat time T3, the current level may be kept at i3 for the period of timeas described supra with respect to FIGS. 3A and 3B before ramping tocurrent level i1.

Alternatively, in step 220, when the number of AB servo marks is to bedifferent from the number of CD servo marks, between time T3(1) andT4(1) or between a time T3(2) or T4(2) on alternating passes through theloop 205, 210, 215, 220, 225 and 230, the current is ramped to currentlevel i1. Alternatively, instead of ramping the current starting at timeT3(1) or T3(2) on alternating passes through the loop 205, 210, 215,220, 225 and 230, the current level may be kept at i3 for the period oftime as described supra with respect to FIGS. 3A and 3B before rampingto current level i1.

In step 225, at time T4 current level i0 is applied. Alternatively, whenthe number of AB servo marks is to be different from the number of CDservo marks, at time T4(1) or T4(2) on alternating passes through theloop 205, 210, 215, 220, 225 and 230 current level i0 is applied.

In step 230, for a time duration T5 the current level is maintained ati0. Note, T1<T2<T3<T4. Alternatively, when the number of AB servo marksis to be different from the number of CD servo marks, at time T5(1) orT5(2) on alternating passes through the loop 205, 210, 215, 220, 225 and230, current level maintained at i0. Note, T1(1)<T2(1)<T3(1)<T4(1) andT1(2)<T2(2)<T3(2)<T4(2).

In step 235, it is determined if writing of servo marks is to bestopped. If no, the method loops back to step 205, otherwise writing ofservo marks is terminated.

FIG. 8 is a flowchart of the method of writing servo marks according tofirst embodiments of the present invention. In the followingdescription, except when referring to numbers of pulses, “negative” maybe substituted for “positive” and “positive” substituted for “negative.”

In step 250, the servo write signal (e.g., 180 of FIG. 4A) is designedto the conditions of TABLE I. The use of a general purpose computer asan aid in the design of the servo write signal is useful. The servowrite signal is supplied to the servo write head (e.g., dual-gap servowrite head 100 of FIG. 1A). During steps 255 through 270, the magnetictape is moving at a constant velocity past the servo write head. Signaldesign techniques include graphical and numeric methods.

In step 255, from a zero or near zero current level i0, a preamble setof M high-frequency pulses of controlled widths and spacings are appliedbetween a time T1 and a time T2. The current level pulses between acurrent level i3 and a current level i4. The trailing edge of the lastpulse ends at current level i4. In one example |i3|=|i4|. In one example|i3|≠|i4|. M is a positive integer equal to or greater than one.Alternatively, in step 255, when the number of AB servo marks is to bedifferent from the number of CD servo marks, a preamble set of M1 pulsesbetween current level i3 and current level i4 are applied between timeT1(1) and T2(1), or a preamble set of M2 pulses between current level i3and current level i4 are applied between time between a time T1(2) and atime T2(2) on alternating passes through the loop 250, 255, 260, 265 and270. M1 may or may not be equal to M2.

In step 260, the current is pulsed N times from i3 to a current level i4until a time T3. In one example |i3|=|i4|. In one example |i3|≠|i4|. Nis a positive integer equal to or greater than one. During time durationT3-T2, pairs of N servo mark are written to the servo track across fromboth gaps. The trailing edge of the last pulse ends at current level i2.

Alternatively, in step 260, the current is pulsed N1 or N2 times fromcurrent level i3 to current level i4 until a time T3(1) or T3(2). N1 andN2 are positive integers equal to or greater than one and N1 is notequal to N2. In one example |i3|=|i4|. In one example |i3|≠|i4|. Duringtime duration T3(1)-T2(1), pairs of N1 servo marks are written to theservo track across from both gaps. During time duration T3(2)-T2(2),pairs of N2 servo marks are written to the servo track across from bothgaps. The trailing edge of the last pulse ends at current level i3. N1pulses or N2 pulses are applied on alternating passes through the loop205,210, 215, 220, 225 and 230.

In step 265, from current level i3, a closing set of P high-frequencypulses of controlled widths and spacings are applied between a time T3and a time T4. The current level pulses between a current level i3 and acurrent level i4. The trailing edge of the last pulse ends at currentlevel i0. In one example |i3|=|i4|. In one example |i3|≠|i4|. P is apositive integer equal to or greater than one. Alternatively, in step265, when the number of AB servo marks is to be different from thenumber of CD servo marks, a closing set of P1 pulses between currentlevel i3 and current level i4 are applied between a time T3(1) andT4(1), or a closing set of P2 pulses between current level i3 andcurrent level i4 are applied between time between a time T3(2) and atime T4(2) on alternating passes through the loop 250, 255, 260, 265 and270. P1 may or may not be equal to P2.

There are multiple permutations of N1, N2, P1, P2, M1 and M2, ten ofwhich are: (1) N1=N2, M1=M2=P1=P2; (2) N1=N2, M1=M2, P1=P2, M1≠P1; (3)N1=N2, M1≠M2, P1=P2; (4) N1=N2, M1=M2, P1≠P2; (5) N1=N2, M1≠M2, P1≠P2;(6) N1≠N2, M1=M2=P1=P2; (7) N1≠N2, M1=M2, P1=P2, M1≠P1; (8) N1≠N2,M1≠M2, P1=P2; (9) N1≠N2, M1=M2, P1≠P2; (10) N1≠N2, M1≠M2, P1≠P2.

In step 270, for a time duration T5 the current level is maintained ati0. Note, T1<T2<T3<T4.

In step 270, for a time duration T5 the current level i0 is maintainedat i0. Alternatively, when the number of AB servo marks is to bedifferent from the number of CD servo marks, for a time duration T5(1)or T5(2) on alternating passes through the loop 250, 255, 260, 265 and270 the current level is maintained at i0.

In step 275, it is determined if writing of servo marks is to bestopped. If no, the method loops back to step 255 otherwise writing ofservo marks is terminated.

The present invention may take the form of first and second apparatuses.The first apparatus, comprising: a servo write head having a first writegap and second write gap spaced apart and an induction coil configuredto generate respective magnetic fields proximate to the first and secondgaps when a current is applied to said coil by a bipolar servo writedriver, the bipolar servo write driver configured to generate bothnegative and positive polarity currents, the bipolar servo write driverconfigured to generate a varying current signal; a component that movesa magnetic storage medium past the first and second write gaps in adirection from the first write gap toward the second write gap; whereinthe varying current signal includes a non-write phase, a preamble phase,a servo mark write phase and a closing phase, the varying current at aninitial current level during the non-write phase, the preamble phasecomprising an abrupt change in current from the initial current level toa first current level and a first current ramp from the first currentlevel to a second current level, the servo mark write phase comprisingcurrent pulses between third and fourth current levels to write servomarks, and the closing phase comprising a second current ramp from thesecond current level to the first current level and an abrupt change incurrent from the first current level to the initial current level; andwherein servo marks are written only during the servo mark write phase.

The second apparatus, comprising: a servo write head having a firstwrite gap and second write gap spaced apart and an induction coilconfigured to generate respective magnetic fields proximate to the firstand second gaps when a current is applied to said coil by a bipolarservo write driver, the bipolar servo write driver configured togenerate both negative and positive polarity currents, the bipolar servowrite driver configured to generate a varying current signal; acomponent that moves a magnetic storage medium past the first and secondwrite gaps in a direction from the first write gap toward the secondwrite gap; wherein the varying current signal comprises a non-writephase, preamble phase, a servo mark write phase and a closing phase,during the non-write phase, the varying current is at an initial currentlevel, the preamble phase comprises a preamble pattern of high-frequencycurrent pulses varying from a first current level to a second currentlevel and back to the first current level, the servo mark write phasecomprises a servo mark pattern of low-frequency current pulsing betweenthe first and second current levels to write servo marks, and theclosing phase comprises a closing pattern of high-frequency currentpulses varying between the first current level to the second currentlevel and back to the first current level; and wherein servo marks arewritten only during the servo mark write phase.

Thus, the embodiments of the present invention provide apparatuses andmethods for bipolar writing servo marks to magnetic storage medium withincreased readback signal amplitude while reducing undesirable pulsesinto the readback signal. It should be understood that while magnetictape has been used in describing the embodiments of the presentinvention, the embodiments of the present invention are applicable toany moving magnetic storage medium.

The description of the embodiments of the present invention is givenabove for the understanding of the present invention. It will beunderstood that the invention is not limited to the particularembodiments described herein, but is capable of various modifications,rearrangements and substitutions as will now become apparent to thoseskilled in the art without departing from the scope of the invention.Therefore, it is intended that the following claims cover all suchmodifications and changes as fall within the true spirit and scope ofthe invention.

1. A method, comprising: providing a servo write head having a firstwrite gap and a second write gap spaced apart and an induction coilconfigured to generate respective magnetic fields proximate to saidfirst and said second write gaps when a current is applied to said coilby a bipolar servo write driver, said bipolar servo write driverconfigured to generate both negative and positive polarity currents;generating a varying current signal using said bipolar servo writedriver; moving a magnetic storage medium past said first and said secondgaps in a direction from said first write gap toward said second writegap; said varying current signal includes a non-write phase, a preamblephase, a servo mark write phase and a closing phase; said varyingcurrent at an initial current level during said non-write phase; saidpreamble phase comprising an abrupt change in current from said initialcurrent level to a first current level and a first current ramp fromsaid first current level to a second current level; said servo markwrite phase comprising current pulses between third and fourth currentlevels to write servo marks; said closing phase comprising a secondcurrent ramp from said second current level to said first current leveland an abrupt change in current from said first current level to saidinitial current level; and wherein servo marks are written only duringsaid servo mark write phase.
 2. The method of claim 1, wherein saidsecond current level is between said first and said third currentlevels.
 3. The method of claim 1, wherein said second current level andsaid third current level are substantially the same current level. 4.The method of claim 1, wherein the magnitude of said second currentlevel is greater than the magnitude of said first current level.
 5. Themethod of claim 1, wherein said third and said fourth current levels areof opposite polarity.
 6. The method of claim 1, wherein magnitudes ofsaid third and said fourth current levels are greater than magnitudes ofsaid initial, said first and said second current levels.
 7. The methodof claim 1, including: simultaneously writing servo marks to differentregions of said servo track of said magnetic storage medium as saidmagnetic storage medium passes said first and said second write gaps. 8.The method of claim 1, including: periodically reducing the magnitude ofsaid varying current to prevent regions of said servo track written bysaid first write gap from being overwritten and/or erased by said secondwrite gap.
 9. The method of claim 1, wherein applying said varyingcurrent includes, in the following order: (a) for a length of time T1,applying said initial current level, said initial current levelcorresponding to a current that does not change the magnetic state ofsaid magnetic medium; (b) abruptly changing from said initial currentlevel to said first current level; (c) over a length of time T2, rampingsaid varying current from said first current level to said secondcurrent level; (d) for a length of time T3, pulsing said varying currentfrom a first polarity to a second polarity and back to said firstpolarity N times to write a set of N servo marks, where N is a positiveinteger equal to or greater than one, said second polarity of anopposite polarity from said first polarity; (e) over a length of timeT4, ramping said varying current from said third current level to saidfourth current level; (f) abruptly changing from said first currentlevel to said initial current level; and repeating (a) through (f)multiple times.
 10. The method of claim 1, wherein applying said varyingcurrent includes, in the following order: (a) for a length of time T1,applying said initial current level, said initial current levelcorresponding to a current that does not change the magnetic state ofsaid magnetic medium; (b) abruptly changing from said initial currentlevel to said first current level; (c) over a length of time T2, rampingsaid varying current from said first current level to said secondcurrent level; (d) for a length of time T3, pulsing said varying currentfrom a first polarity to a second polarity and back to said firstpolarity N times to write a set of N servo marks, said second polarityof an opposite polarity from said first polarity; (e) over a length oftime T4, ramping said varying current from said third current level tosaid fourth current level; (f) abruptly changing from said first currentlevel to said initial current level; and repeating (a) through (f)multiple times wherein N alternates between two different positiveintegers that are greater than one and (i) T1, T2, T3 and T4 remainsubstantially the same, (ii) T1, T2, T3 and T4 each alternates between arespective pair of different values, or (iii) one or more of T1, T2, T3and T4 remain substantially the same and one or more of T1, T2, T3 andT4 alternates between a respective pair of different values.
 11. Themethod of claim 1, including: wherein the waveform of said varyingcurrent signal is based on the inequalities:d3ab<Sg−d2ab−Wl−[(Sh/2)*((tan(Al)+tan(Ar))];d1cd<AC−Sg−d2ab−[(Sh/2)*((tan(Al)+tan(Ar))];d3cd<Sg−d2cd−Wl−[(Sh/2)*((tan(Al)+tan(Ar))];d1ab<CA−Sg−d2cd−[(Sh/2)*((tan(Al)+tan(Ar))]; where:AC=d2ab+d3ab+d4ab+d1cd; CA=d2cd+d3cd+d4cd+d1ab; Sg is thecenter-to-center distance between the left and right gaps; Wl is thewidth of the left gap in the X-direction; Wr is the width of the rightgap in the X-direction; Al is the angle of the left gap slanted awayfrom the Y-direction; Ar is the angle of the right gap slanted away fromthe Y-direction; d1ab, d2ab, d3ab, d4ab, d1cd, d2cd, d3cd and d4cd aresequential segments along the magnetic tape in the X-direction where: insegment d1ab the current ramps negative; in segment d2ab the currentpulses from negative to positive and back to negative N1 times, where N1is a positive integer equal to or greater than one; in segment d3ab thecurrent ramps positive; in segment d4ab the current does not change themagnetic state of said magnetic medium; in segment d1cd the currentramps negative; in segment d2cd the current pulses from negative topositive and back to negative N2 times, where N2 is a positive integerequal to or greater than one and N1 is not equal to N2; and in segmentd3cd the current ramps positive; in segment d4cd the current is a zerocurrent or a current that does not change the magnetic state of saidmagnetic medium; and Sh is the width of the servo track in theY-direction; and the X-direction is defined as the direction of movementof the magnetic storage medium from the right gap to the left gap, andthe Y-direction is defined as a direction in a plane of said magneticstorage medium perpendicular to the X-direction.
 12. A method,comprising: providing a servo write head having a first write gap and asecond write gap spaced apart and an induction coil configured togenerate respective magnetic fields proximate to said first and saidsecond gaps when a current is applied to said coil by a bipolar servowrite driver, said bipolar servo write driver configured to generateboth negative and positive polarity currents; generating a varyingcurrent signal using said bipolar servo write driver; moving a magneticstorage medium past said first and said second write gaps in a directionfrom said first write gap toward said second write gap; said varyingcurrent signal comprises a non-write phase, preamble phase, a servo markwrite phase and a closing phase; during said non-write phase, saidvarying current is at an initial current level; said preamble phasecomprises a preamble pattern of high-frequency current pulses between afirst current level and a second current level and back to said firstcurrent level; said servo mark write phase comprises a servo markpattern of low-frequency current pulses between said first current leveland said second current level and back to said first current level forwriting servo marks; said closing phase comprises a closing pattern ofhigh-frequency current pulses between said first current level and saidsecond current level and back to said first current level; and whereinservo marks are written only during said servo mark write phase.
 13. Themethod of claim 12, wherein said first and said second current levelsare of opposite polarity.
 14. The method of claim 12, wherein widths ofpulses and distances between pulses of said servo mark pattern oflow-frequency pulses are larger than the widths of pulses and distancesbetween pulses of said preamble and closing patterns of high-frequencypulses.
 15. The method of claim 12, wherein widths of pulses anddistances between pulses of said preamble pattern of high-frequencypulses are different than widths of pulses and distances between pulsesof said closing pattern of high-frequency pulses.
 16. The method ofclaim 12, including: simultaneously writing servo marks to differentregions of said servo track of said magnetic storage medium as saidmagnetic storage medium passes said first and second write gaps.
 17. Themethod of claim 12, including: periodically reducing the magnitude ofsaid varying current to prevent regions of said servo track written bysaid second write gap from being overwritten and/or erased by said firstwrite gap.
 18. The method of claim 12, wherein applying said varyingcurrent includes, in the following order: (a) for a length of time T1,applying said initial current level, said initial current correspondingto a current that does not change the magnetic state of said magneticmedium; (b) for a length of time T2 applying M pulses of said preamblepattern of high-frequency current pulses where M is a positive integerequal to or greater than one; (c) for a length of time T3, pulsing saidvarying current from a first polarity to a second polarity and back tosaid first polarity N times to write a set of N servo marks, where N isa positive integer equal to or greater than one, said first polarity ofan opposite polarity from said first polarity; (d) for a length of timeT4 applying P pulses of said closing pattern of high-frequency currentpulses, where P is a positive integer equal to or greater than one; andrepeating (a) through (d) multiple times.
 19. The method of claim 16,wherein applying said varying current includes, in the following order:(a) for a length of time T1, applying said initial current level, saidinitial current corresponds to a current that does not change themagnetic state of said magnetic medium; (b) for a length of time T2applying M pulses of said preamble pattern of high-frequency currentpulses where M is a positive integer equal to or greater than one; (c)for a length of time T3, pulsing said varying current from a firstpolarity to a second polarity and back to said first polarity N times towrite a set of N servo marks, where N is a positive integer equal togreater than one, said first polarity of an opposite polarity from saidfirst polarity; (d) for a length of time T4 applying P pulses of saidclosing pattern of high-frequency current pulses, where P is a positiveinteger equal to or greater than one; and repeating (a) through (d)multiple times: wherein N alternates between two different positiveintegers that are greater than one, wherein (i) M and P remain the same,(ii) M and P each alternates between a respective pair of differentvalues, or (iii) M or P alternates between a respective pair ofdifferent values; and wherein (iv) T1, T2, T3 and T4 remainsubstantially the same, (v) T1, T2, T3 and T4 each alternates betweenrespective pairs of different values, or (vi) one or more of T1, T2, T3and T4 remain substantially the same and one or more of T1, T2, T3 andT4 alternates between respective pairs of different values.
 20. Themethod of claim 12, including: wherein the waveform of said varyingcurrent signal is based the inequalities:d3ab<Sg−d2ab−Wl−[(Sh/2)*((tan(Al)+tan(Ar))];d1cd<AC−Sg−d2ab−[(Sh/2)*((tan(Al)+tan(Ar))];d3cd<Sg−d2cd−Wl−[(Sh/2)*((tan(Al)+tan(Ar))];d1ab<CA−Sg−d2cd−[(Sh/2)*((tan(Al)+tan(Ar))]; where:AC=d2ab+d3ab+d4ab+d1cd; CA=d2cd+d3cd+d4cd+d1ab; Sg is thecenter-to-center distance between said first and second gaps; Wl is thewidth of said second write gap in an X-direction; Wr is the width ofsaid first write gap in said X-direction; Al is the angle the secondwrite gap slanted away from a Y-direction; Ar is the angle of the firstwrite gap slanted away from said Y-direction; d1ab, d2ab, d3ab, d4ab,dicd, d2cd, d3cd and d4cd are sequential segments along the magnetictape in the X-direction where: in segment d1ab there are M1 pulses of afirst preamble pattern of high-frequency current pulses where M1 is apositive integer greater than or equal to one; in segment d2ab there areN1 pulses of a first servo mark pattern of low-frequency current pulseswhere N1 is a positive integer greater than or equal to one; in segmentd3ab there are P1 pulses of a first closing pattern of high-frequencycurrent pulses where P1 is a positive integer greater than or equal toone; in segment d4ab the current is zero or near zero; in segment d1cdthere are M2 pulses of a second preamble pattern of high-frequencycurrent pulses where M2 is a positive integer greater than or equal toone; in segment d2cd there are N2 pulses of a second servo mark patternof low-frequency current pulses where N2 is a positive integer greaterthan or equal to one and N1 is not equal to N2; in segment d3cd thereare P2 pulses of a second closing pattern of high-frequency currentpulses where P2 is a positive integer greater than or equal to one; insegment d4cd the current is zero or near zero; and Sh is the width ofthe servo track in the Y-direction; and said X-direction is defined asthe direction of movement of the magnetic storage medium from said firstwrite gap to said second write gap and the Y-direction is defined as adirection in a plane of said magnetic storage medium perpendicular tosaid X-direction.